Every car that I have drivent hat needed new shocks, was very firm over bumbs. you'd hit a bump and bounce sharply for a bit. if thats the problem you are having, its probably shocks.
Have you done the push test?
Push down on one of the rear corners and release, if the car returns to normal without bouncing up and down, the shocks are fine. if it bounces, get em replaced or checked.
I don't think springs fail that often if at all.
I'm not a tech, so don't take my word for it. justa way you can tell if the shocks are gone or not.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.